In 1967, Woese classified methods of interpreting the genetic (or amino acid) code in mechanistic and stochastic models. In his mechanistic model Woese attempted a molecular geometrical fit between codons and their coded amino acids. One of the first attempts at such a code was the "diamond" code proposed by Gamow in 1954. As recently as 1981, when the actions of tRNA and rRNA in the synthesis of protein were already well-known, the geometrical method still attracted attention, as typified by the work of Hendry et al. (1981) who drew a structural analogy between codons and amino acids. However, the lack of any direct contact between DNA or mRNA with protein molecules rendered "lock and key" models impotent as predictors of coding mechanisms.
Gamow's attempt is an admirable one, because, although his "diamond" geometry was discredited by later observation, he did offer a combinatorial schema motivated by the few numbers available at that time, i.e., four nucleotide bases, 64 triplet-base codons with only 20 amino acids. Unfortunately his combinatorics and geometry could not be separated from each other, and the combinatorial part suffered the same fate as his proposed geometric model.